Recently, from a viewpoint in which a weight saving, a streamlined design, and a miniaturization of a bushing, a shared using of the kind of the bushing, and a simplification of a working process are attempted, a polymer bushing of a solid insulation structure (completely dry type) which is directly molded onto the surface of the insulator such as the epoxy bushing by the polymer covering such as the silicone rubber is used (for example, refer to Patent document No. 1).
However, in the polymer bushing of such composition, because corona discharge occurs in an outer surface of the polymer bushing when electric field becomes high, there is a difficult point that the polymer covering deteriorates (erosion) by chemical erosion when aforementioned corona discharge occurs for long period.
For preventing the occurrence of such corona discharge, it is necessary to reduce electric field strength on surface of the polymer bushing in air. As the method for reducing such the electric field strength, (1) the method of providing the electric-field stress-control layer at the interface between the insulator and the polymer covering is known (for example, refer to Patent document No. 2), and (2) the method of thickening an outer diameter of the polymer covering is known (for example, refer to Patent document No. 3).
However, in the method of (1), although heat generation does not occur in an operating voltage of hitherto known voltage specification (66/77 kV), for example, when the operating voltage rises to about 154 kV, because the electric-field stress-control layer generates heat by switching in the aforementioned operating voltage, there is a difficult point that the problem of deterioration cannot ignore.
Here, it is understood that the above-described exothermic phenomenon is caused by the volt-ampere characteristic of the electric-field stress-control layer. That is, as shown in FIG. 4, the volt-ampere characteristic of the electric-field stress-control layer is nonlinear, and the electric current hardly flows at the operating voltage of about 66-77 kV (A part in drawing). However, the large current flows into the electric-field stress-control layer in the part that the electric field concentrates when the operating voltage exceeds about 154 kV (B part in drawing). That is, by aforementioned current, as shown in FIG. 5, it is understood that the heat generation occurs in the electric-field stress-control layer in the neighborhood of a shielding metal fitting 400 of the polymer bushing. In FIG. 5, numeral 100 shows a conductor bar, numeral 200 shows the insulator, numeral 300 shows the polymer covering, numeral 310 shows shades of the polymer covering, and numeral 500 shows the electric-field stress-control layer.
Meanwhile, in the method of (2), although it is possible to reduce the electric field strength of the surface of the polymer bushing (the part of the shade 310 of the polymer covering 300) to a certain degree, for obtaining the electric field strength which is equivalent to the method of (1), the outer diameter of the polymer covering 300 must be made large more than required. Therefore, in the method of (2), the deterioration of brine damage characteristic is caused by increase of surface area of an indoor/outdoor termination when the outer diameter of the polymer covering 300 becomes larger. And, there are difficult points that the increase of weight and the increase of cost of the cable sealing end are caused by needing more insulating materials.    [Patent document No. 1] Patent publication number 2003-303632    [Patent document No. 2] Patent publication number 2005-117806    [Patent document No. 3] Patent publication number 2002-157932